France has opened an inquiry into a video purporting to show the killing by Islamic State (IS) of an Israeli Arab, which shows a French-speaking militant.

The militant speaks in the video as a child seems to shoot the victim, whose family deny he was an Israeli spy.

French officials say the militant is probably Sabri Essid, a half-brother of Mohammed Merah, the Islamist who shot dead seven people in France in 2012.

French media reports say the boy in the video could be Merah’s stepson.

French prosecutors told the BBC they believed both the man and the boy in the IS video were French.

The officials said they had decided to launch an investigation to determine their identities for sure.

Under-18s have regularly appeared in militant videos, sometimes committing violent acts, and the group has actively encouraged children to train as fighters, our Paris correspondent, Lucy Williamson, reports.

Another French recruit whose death was announced this week by IS was said to be a young teenager from Strasbourg, she adds.

In the video released last week, the man standing in the background – thought to be Mr Essid – can be heard making threats to Jewish people.

He is thought to be a half-brother of Merah, who killed three unarmed French soldiers, a rabbi and three small children at a Jewish school in south-western France three years ago.

Merah, who was 23, was then shot dead by police in a siege at his home.